In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may service wireless communication devices (WCDs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality. In general, each coverage area may operate on one or more carriers each defining a respective bandwidth of coverage, and each coverage area may define an air interface providing a downlink for carrying communications from the base station to WCDs and an uplink for carrying communications from WCDs to the base station. The downlink and uplink may operate on separate carriers or may be time division multiplexed over the same carrier(s). Further, the air interface may define various channels for carrying communications between the base station and WCDs. For instance, the air interface may define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. On each such carrier used for downlink communications, the air interface then defines a Physical Downlink Shared Channel (PDSCH) as a primary channel for carrying data from the base station to WCDs, and a Physical Downlink Control Channel (PDCCH) for carrying control signaling from the base station to WCDs. Further, on each such carrier used for uplink communications, the air interface defines a Physical Uplink Shared Channel (PUSCH) as a primary channel for carrying data from WCDs to the base station, and a Physical Uplink Control Channel (PUCCH) for carrying control signaling from WCDs to the base station.
In LTE, downlink air interface resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines a continuum of 10-millisecond (ms) frames, divided into 1-ms sub-frames and 0.5-ms slots. With this arrangement, each sub-frame is considered to be a transmission time interval (TTI). Each frame has 10 sub-frames (or TTIs), and each sub-frame has 2 slots. In the frequency domain, on the other hand, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), to form one OFDM symbol.
LTE further defines a particular grouping of time-domain and frequency-domain resources as a downlink resource block. In the time domain, each downlink resource block has a duration corresponding to one slot (0.5 ms). In the frequency domain, each downlink resource block consists of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 0.5 ms duration of a downlink resource block accommodates 7 OFDM symbols, each spanning 66.7 microseconds, with a 4.69-microsecond guard band (cyclic prefix) added to help avoid inter-symbol interference. Depending on the bandwidth of the downlink carrier, the air interface may support transmission on a number of such downlink resource blocks in each slot. For instance, a 5 MHz carrier supports 25 resource blocks in each slot, whereas a 15 MHz carrier supports 75 resource blocks in each slot.
The smallest unit of downlink resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a resource block that consists of 12 sub-carriers and 7 OFDM symbols has 84 resource elements. Further, each OFDM symbol and thus each resource element can represent a number of bits, with the number of bits depending on how the data is modulated. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each modulation symbol may represent 2 bits; with 15 Quadrature Amplitude Modulation (16QAM), each modulation symbol may represent 4 bits; and with 64QAM, each modulation symbol may represent 6 bits.
Within a resource block, and cooperatively across all of the resource blocks of the carrier bandwidth, different resource elements can have different functions on the downlink. In particular, certain resource elements (e.g., 8 resource elements distributed throughout the resource block) may be reserved for reference signals used for channel estimation. In addition, certain resource elements (e.g., resource elements in the first one, two, or three OFDM symbols per TTI) may be reserved for the PDCCH and other control channels (e.g., a physical hybrid automatic repeat request channel (PHICH)), and most of the remaining resource elements (e.g., most of the resource elements in the remaining OFDM symbols per TTI) would be left to define the PDSCH.
Across the carrier bandwidth, each TTI of the LTE air interface thus defines a control channel space that generally occupies a certain number of 66.7-microsecond symbol time segments (e.g., the first one, two, or three such symbol time segments), and a PDSCH space that generally occupies the remaining symbol time segments, with certain exceptions for particular resource elements. With this arrangement, in the frequency domain, the control channel space and PDSCH space both span the entire carrier bandwidth. In practice, the control channel space is then treated as being a bandwidth-wide space for carrying control signaling to WCDs. Whereas, the PDSCH is treated as defining discrete 12-sub-carrier-wide PDSCH segments corresponding to the sequence of resource blocks across the carrier bandwidth.
One of the main functions of the PDCCH is to carry Downlink Control Information (DCI) messages to served WCDs. LTE defines various types or “formats” of DCI messages, to be used for different purposes, such as to indicate how a WCD should receive data in the PDSCH of the same sub-frame, or how the WCD should transmit data on the PUSCH in an upcoming sub-frame. For instance, a DCI message in a particular sub-frame may schedule downlink communication of bearer data to a WCD, by specifying one or more particular resource blocks that carry the bearer data in the sub-frame's PDSCH, what modulation scheme is used for the downlink transmission, and so forth.
Each DCI message may span a particular set of resource elements on the PDCCH (e.g., one, two, four, or eight control channel elements (CCEs), each including 36 resource elements) and may include a cyclic redundancy check (CRC) that is masked (or scrambled) with an identifier (e.g., a particular radio network temporary identifier (RNTI)). In practice, a WCD may monitor the PDCCH in each sub-frame in search of a DCI message having one or more particular RNTIs. And if the WCD finds such a DCI message, the WCD may then read that DCI message and proceed as indicated. For instance, if the DCI message schedules downlink communication of bearer data to the WCD on particular PDSCH resources in the current sub-frame, the WCD may then read the indicated PDSCH resources to receive that bearer data.